Revolving doors have long been useful in providing ingress and egress to buildings while minimizing the temperature differential losses from the building's interior to exterior ambient air. One concern with the use of revolving doors is that in case of fires or other emergencies, the capacity to let people out of the building is limited. Much of the frontage traversed by the door cannot be used to permit escape from the building. This is recognized in building codes requiring exit door frontage in addition to that provided by any revolving doors.
To increase the ability to allow people to escape in a panic situation, revolving doors having collapsible wings have been used. Typically, such doors have been designed for an individual to purposefully actuate an emergency breakaway option integrated into the revolving door.
A problem which has existed with collapsible revolving doors is that of stack pressure. That is the difference in air pressure created between the exterior of the building and the air within the building. There is generally a temperature difference between that of the air inside and outside. This is as a result of the external ambient environment and the space conditioning system within the building, either making the interior at a warmer or cooler temperature than the outside ambient conditions. The greater density of the colder air with respect to that of the warmer air causes the pressure differential. When the door rotates, only a small portion of the air in the building escapes. In fact, this indeed is one of the advantages of using revolving doors as it entrains the internal ambient environment to a great extent, thus economizing on energy costs by preventing escape of the internal ambient environment from the building.
But there is a sensitivity problem. The doors must be adjusted so that the collapsing feature may be actuated by even the frailest of individuals, yet it must not be actuated in non-panic situations. The collapsing feature must not require active thinking of the person seeking egress from the building. The stack pressure referred to above creates a problem, because it can be sufficient to actuate the collapsing feature. But if the sensitivity to the pressure is reduced, then the weak individual will be unable to cause the collapsing feature to actuate in an emergency situation.
Electromagnetic latches such as that described in U.S. patent application Ser. No. 374,899, filed 4 May 1982, provide a solution to the above problems and allow for remote (electrical) actuation of the latch. Electrical devices must have backup battery power supples to be operable during a power failure so power consumption is a major consideration. To reduce power draw the magnet can be made larger, and thus more efficient. Size eventually becomes a production and installation problem, however. Therefore the optimum magnetic latch has a very high resistance to horizontal shear force, is small, and uses little energy when energized.